The invention relates to the field of communications, and more particularly to techniques for generating and managing precision frequency sources in cellular telephones or other communications devices having a location capability, such as Global Positioning System (GPS) or other location service.
Two important performance metrics for any GPS or other location-reporting services include the time needed to acquire synchronization with a signal source, and the ability to detect weak signals in noise. For GPS receivers these metrics correspond to time to first fix (TTFF) and receiver sensitivity, respectively. In a practical GPS receiver these metrics are dependent on the availability of an accurate frequency reference to drive the GPS receiver. Accuracies on the order of 0.5 ppm or better are required to attain acceptable GPS performance, for example TTFF ranges of a few tens of seconds. Conventional implementations require expensive precision components such as a temperature compensated crystal oscillator (TXCO) or oven controlled crystal oscillator (OCXO) in order to achieve this level of accuracy.
As a result of the FCC-mandated E911 location service, GPS receivers are being integrated into cellular phones. Cellular networks use highly accurate clocks to maintain network synchronization. Cellular handsets typically contain their own stable reference clock which is locked to the cellular network by automatic frequency control (AFC) or other circuits. However the resulting frequency reference for cellular communications is generally different than that needed for GPS downconversion or other GPS operations. Other problems exist.
The invention overcoming these and other problems in the art relates in one regard to a system and method for frequency management in a communications device having a positioning feature, such as a cellular phone equipped with GPS location capability, which can dynamically detect the error in a GPS receiver""s reference oscillator without directly correcting that oscillator, but instead adjusting Doppler search or other control logic on the GPS side. This type of frequency aiding may be applied continuously or periodically to maintain very accurate frequency information, allowing a narrower bandwidth correlation to be used thereby improving the signal to noise ratio (SNR), and hence, sensitivity of the GPS receiver in a hybrid communications/location device. The frequency assist information may both narrow the carrier offset search space required during satellite acquisition thereby reducing the TTFF of a GPS receiver, as well as shorten the duration which the RF receiver must be powered on thereby reducing the power consumption of a GPS receiver, extending its battery life. Since existing cellular circuitry is made use of to enhance GPS operation, less expensive components may be used, enabling a cost reduction in an integral cellular handset/GPS receiver.